1. Field of the Invention
The present invention relates to hearing aids. The invention more particularly relates to hearing aids that rely on adaptive feedback suppression in order to reduce the problems caused by acoustic and mechanical feedback. The invention further relates to a method of suppressing feedback in a hearing aid. More specifically, the invention relates to methods for control of the adaptation of feedback suppression systems in hearing aids.
As used within the context of this disclosure, a hearing aid is a small portable device adapted for providing at the users eardrum a version of the acoustic environment that has been amplified according to the users prescription. The device usually comprises a microphone, an amplifier and a miniature loudspeaker (in the hearing aid parlance referred to as a “receiver”) situated in an earpiece placed in the users ear canal. It is well known that the operation may suffer from feedback problems, due to the amplified sound feeding back from the speaker to reach the microphone, where it will be picked up and amplified again. Feedback may be acoustic, i.e. through the air around the earpiece, or mechanical, i.e. through the structure of the earpiece.
Acoustic and mechanical feedback from a receiver to one or more microphones will limit the maximum amplification that can be applied in a hearing aid. Due to the feedback, the amplification in the hearing aid can cause resonances, which shape the spectrum of the output of the hearing aid in undesired ways, and, even worse, it can cause the hearing aid to become unstable, resulting in whistling or howling. The hearing aid usually employs compression to compensate hearing loss; that is, the amplification gain is reduced with increasing sound pressures. Moreover, an automatic gain control is commonly used on the output to limit the output level, thereby avoiding clipping of the signal. In case of instability, these compression effects will eventually make the system marginally stable, thus producing a howl or whistle of nearly constant sound level.
Feedback suppression is often used in hearing aids to compensate the acoustic and mechanical feedback. The acoustic feedback path can change dramatically over time as a consequence of, for example, amount of earwax, the user putting on a hat or holding a telephone to the ear, or the user chewing or yawning. For this reason it is customary to apply an adaptation mechanism on the feedback suppression to account for the time-variations.
An adaptive feedback suppression filter can be implemented in a hearing aid in several different ways. For example, it can employ an IIR filter, an FIR filter, or a combination of the two. It can be composed of a combination of a fixed filter and an adaptive filter. The adaptation mechanism can be implemented in several different ways, for example algorithms based on Least Mean Squares (LMS), Normalized Least Mean Squares (NLMS) or Recursive Least Squares (RLS).
2. The Prior Art
WO-A1-2007113282 discloses an anti-feedback system where the adaptation of the coefficients of the adaptive feedback cancellation filter is halted if it is detected that an external tone is played. The publication also discloses methods and hearing aids adapted to detect if a sudden increase in sound pressure occurs and to temporarily suspend the adaptation of the feedback cancellation filter afterwards.
WO-A1-2007113282 discloses that loud sounds (not necessarily sudden) can also cause a nonlinear behavior in one or more components of the hearing aid. The acoustic feedback path as it is seen from the cancelling filter's perspective embraces microphone(s), receiver and input- and output converters. Saturation or overload in one of these units thus corresponds to a non-linearity in the acoustic feedback path. Assuming a linear filter is used for feedback cancellation (such as an FIR filter), the filter is inadequate for modeling the highly nonlinear saturation function, thus leading to errors in the adaptation. Therefore, according to an embodiment, a detector for recognition of these circumstances is included in the adaptation mechanism, and adaptation of the cancellation filter is temporarily suspended when the non-linearity occurs. The adaptation may, according to a particular embodiment, be suspended for a short while after one occurrence of that kind has been detected.
Generally WO-A1-2007113282 discloses that in order to maintain a specific uncertainty on the filter coefficients, the adaptation speed should be reduced when the gain of the hearing aid is reduced.
U.S. Pat. No. 6,434,247 discloses a feedback cancellation system for a hearing aid that adapts a first filter in the feedback path that models the quickly varying portion of the hearing aid feedback path, and adapts a second filter in the feedback path that is used either as a reference filter for constrained adaptation or to model more slowly varying portions of the feedback path. The second filter is updated only when the hearing aid signals indicate that an accurate estimate of the feedback path can be obtained. Changes in the second filter are then monitored to detect changes in the hearing aid feedback path. The first filter is adaptively updated at least when the condition of the signal indicates that an accurate estimate of physical feedback cannot be made. It may be updated on a continuous or frequent basis. It is further disclosed that in a compression hearing aid, the lower the ambient signal level the higher the gain, resulting in a more favorable level of the feedback relative to that of the ambient signal at the microphone and hence giving better convergence of the adaptive filter and a more accurate feedback path model. Thus the rate of adaptation in a compression hearing aid can be increased at low input signal levels or equivalently for high compression gain values.
One problem with the prior art systems is that the anti-feedback system during unfavorable conditions will attempt to adapt to an input signal that is primarily noise.
Another problem is that the anti-feedback system during unfavorable conditions may adapt so fast that the hearing aid becomes unstable.
It is therefore a feature of the present invention to overcome at least these drawbacks and provide more efficient and stable methods for adaptation of anti-feedback systems in hearing aids while maintaining the sound fidelity of the acoustical sounds. Hereby user comfort for the hearing impaired can be improved.
It is another feature of the present invention to provide a hearing aid comprising an anti-feedback system with improved user comfort and sound fidelity.